May 4, 2009 (CIDRAP News)  The tentative detection of the novel swine influenza H1N1 virus in an Alberta swine herd over the weekend shook Canada's pork industry and raised concern about the potential for new hybrid viruses to emerge.

Canadian authorities said on May 2 that preliminary testing detected the virus in an Alberta herd and that it probably came from a Canadian carpenter who works on the farm and had a flu-like illness when he returned from a visit to Mexico in mid-April

Dr. Brian Evans of the Canadian Food inspection Agency (CFIA) said the worker had contact with the pigs on Apr 14 and that about 220 pigs in the herd of 2,200 began showing signs of sickness on Apr 24, according to a Canadian Press report.

The carpenter has recovered and the pigs were recovering, the CFIA said. The farm has been quarantined.

A US Department of Agriculture (USDA) statement said the worker's family also had a flu-like illness but were recovering. The agency said it would take anywhere from 5 days to 2 weeks to get the final test results on the Canadian herd.

Pigs are often infected with flu viruses, including strains from humans and birds. They are described as a mixing vessel where different viruses can trade genes (reassort) and produce new variants. The novel H1N1 virus itself has been said to contain genetic material from swine, avian, and human flu viruses.

"Reassortment is a concern that people express because pigs have their own influenza virus, so if they get infected with this [human] one, do you have to worry about other reassortant viruses coming out? Yes, it seems that would be a risk," said David A. Halvorson, PhD, a veterinarian and avian influenza expert at the University of Minnesota in St. Paul.

Global animal and human health officials said the Canadian finding is not a big surprise.

"The human-to-animal transmission that occurred in Canada does not come as a surprise as influenza viruses are capable of transmitting from humans to animals," said Joseph Domenech, chief veterinary officer of the United Nations Food and Agriculture Organization (FAO) said in a statement today.

At a news briefing yesterday, Peter Ben Embarek of the World Health Organization (WHO) said the virus isolated from the swine does not appear to differ from the virus spreading among humans. "There is no sign that it has changed at all. But this could of course happen like with any other flu viruses," he said. He added that it's important to increase surveillance in humans and animals so as to detect any mutations.

The Canadian finding added to fears in some quarters about the safety of pork, given that the illness caused by the new virus infecting people in 21 countries is unofficially called swine flu.

The US Department of Agriculture (USDA) said pork and pork products remain safe and the finding would not affect US trade with Canada.

Humans occasionally pick up influenza viruses from pigs. Reports of human-to-pig transmission are apparently rare, but such cases are assumed to happen.

"We really don't know how common that might be; there's no way of knowing that," said Halvorson. "We do know that pigs have been harboring a virus that has genes form human viruses in it. Those genes had to come from someplace, and presumably they came from a human flu virus getting into pigs." He said the virus, an influenza A/H3N2 subtype, has been found in pigs in many parts of the United States.

The FAO advised that when swine show signs of respiratory illness, operators should use strict biosecurity measures, including restricting the movement of pigs, goods, and people.

Also, people who work with pigs should not go to work if they have any signs of respiratory disease, fever, or any flu-like illness, the FAO said. At the same time, the agency said there is "absolutely no need to slaughter animals" to prevent the spread of the novel H1N1 virus.

Meanwhile, the USDA said it is "actively working to develop an H1N1 vaccine for swine, just as the CDC is doing for humans."

At the WHO briefing, Ben Embarek said at least two laboratories are experimentally exposing pigs to the new virus to see how it affects them. Also, the World Organization for Animal Health (OIE) said today it is awaiting the results of experiments to determine the susceptibility of various animals to the H1N1 virus.